step-down vs diminution

step-down and diminution both are nouns.

Word	Noun	Adjective	Verb	Adverb
step-down	Yes	No	No	No
diminution	Yes	No	No	No

As nouns, step-down and diminution are synonyms defined as:

Other synonyms of step-down include decrease, reduction.

step-down (noun)	diminution (noun)
the act of decreasing or reducing something	the act of decreasing or reducing something
	the statement of a theme in notes of lesser duration (usually half the length of the original)
	change toward something smaller or lower